Machine structural parts, for example, gears of automatic transmissions or sheaves of continuously variable transmissions, constant velocity joints, hubs, and other power transmission parts, are required to have surface fatigue strength. In the past, generally the above parts were made from a material of case hardened steel such as JIS SCr420, SCM420, etc. having a C of around 0.2% and were hardened by carburization to obtain a martensite structure having a C at the surface of the part of around 0.8% so as to improve the surface fatigue strength.
In recent years, along with the increasing stringency of the usage conditions, improvement in the surface fatigue strength has been sought. In place of the conventional carburization hardening, carbonitridation with its more superior softening resistance is being focused on.
For example, Japanese Patent Publication (A) No. 7-190173 proposes nitridation or carbonitridation to make the nitrogen content 0.2% to 0.8%, then salt dipping for quenching for obtaining a higher strength and furthermore shot peening after the above treatment so as to further increase the strength.
Further, Japanese Patent Publication (A) No. 2006-292139 proposes carbonitridation and annealing, then induction hardening to provide a surface part with an amount of residual austenite of 15 vol % or more and thereby improve the peeling lifetime of a pinion shaft.
However, in recent years, along with further tougher usage conditions, under usage conditions, the operating surfaces have become stronger than 300° C. and weaker than 400° C. or much higher in temperature and therefore further improvement in the surface fatigue strength has been sought.